Building Tips
Extra 330-S
Guidance Notes forBuilders & Pilots of Giant Models.
By Clive Butler.
(original text from VFSAA Newsletter)
These notes are an attempt to provide the modeller proposing to build a
Giant model with some guidance on the
requirements of MAAA Manual Of Procedures MOP015
relating to the
inspection and certification
process.
Although this process may, at times,
appear onerous please accept that it is formulated with the safety
(both of people and the model) in mind and is not
a deliberate attempt to make life difficult. Please also accept that
the Inspector you have chosen, not only has your
best interests in mind, but is also legally obliged to follow the MOP.
Cutting corners is not an acceptable approach to
producing a safe aircraft with any hope of longevity.
Steps in the certification
process.
Step 1.
Assessment of building drawings/ARF kit/pre-built model and
determination of building inspection schedule
based on the experience of the modeller and quality of design.
Step 2.
Assessment of the proposed electronic equipment, control
linkages and engine suitability.
Step 3.
Carry out scheduled inspections during construction.
Step 4.
Carry out pre-flight inspection.
Step5 .
Observe and log test flights (min. 3)
It is critical that any modeller intending to build a model that will
have a dry mass of 25Kgs or more contacts
an MAAA Heavy Model Inspector BEFORE commencing construction of the
model. The Giant Model Inspector
shall assess the building drawings, ARF kit or prebuilt model, to
determine when the inspection schedule should
commence, taking into account the degree of complexity of the project.
(MOP015 - 6.1.2.1)
The number and timing of
the inspection program should be based on the experience of the
modeller and whether or not it is a "Proven"
design.
(an enlarged commercially available plan shall be treated as an
"Own Design"). (MOP015 - 6.1.2.2)
DESIGN.
Consideration of the design should include general design concepts such
as wing incidents, decalage, washout and
thrust, as well as construction methods, materials, fasteners and
fittings to be used.
STRUCTURE.
Wings.
It is necessary to consider what forces will act upon them in flight
and on landing.
Biplanes - The wings should be considered together as a unified
structure.
This means that the integrity of the
interplane struts and wires is fundamental. Correspondingly the spars
can be much lighter than those of a cantilever
monoplane.
Cantilever monoplanes - Here the load bearing structure is quite
different. The strength of the spar will be crucial. It
should be carefully determined at and near the centre section. The
upper elements of the spar will be in compression
during level flight and the lower component will be in tension. Their
cross section should be adequate and the
materials should be suitable.
Consideration should be given to the
loads imposed on landing by the undercarriage (if
mounted in the wing) as well as inertia.
Multi engines - Where two or more engines are mounted on the wings they
will alter the loads. Not only will they
contribute a large additional down force on the wings when landing, but
if they have substantial forward overhang,
they will cause considerable torsional force.
Struts and wires - These should be fully load bearing. In most cases
struts have to perform in compression as well as
tension and should not bow in compression. The attachment points for
both struts and wires should be designed into
the structure to adequately spread the load.
Tail Group.
The most likely problem to arise in the tail group is flutter. This may
be caused if the construction is too light or by
unbalanced surfaces. Heavy elevators and/or rudder hinged at their
leading edge with no counter balance should be
avoided.
Fuselage.
Structural failure of the fuselage is not common, however particular
attention should be paid to the area where the
engine/s are attached and their method of attachment. The use of
flexible mounts is somewhat contentious, so if there is
any doubt it is probably safer to mount them rigidly.
Control Surfaces.
These should be light but rigid. Small gaps, strong hinges and horns
locked into place. Consider static or dynamic
balance for the surface.
Control Linkages.
The control linkages, clevises and horns must be able to withstand the
maximum torque output of the servo. The
minimum dia. for pushrod ends used on primary control surfaces should
be 4-40. Pull/pull systems are recommended
where appropriate and heavy duty servo arms must be used. (MOP015
6.3.1.5)
Adhesives.
There are a large number of very good adhesives available for various
applications. The builder should discuss what he
intends to use for different jobs. As a general rule, tension should
not be placed on an adhesive and where necessary
this should be avoided by the use of screws, bolts or gussets to give
additional support to the joint.
In particular the
engine bulkhead (firewall) must be properly attached employing screws
or well engineered joints. Where there is
tension that would tend to delaminate ply there should be adequate
bolts that go through the whole thickness.
ARF models.
Because it may be difficult (if not impossible) for the inspector to
determine aspects of the construction undertaken
by the manufacturer at the factory, ARF models must be
inspected to the maximum extent possible, and ensure that
adequate test flying is carried out to confirm the structural
airworthiness of the model (MOP015 - 6.1.2.4)
CONTROL SYSTEMS
In general the equipment must be of high quality and principles of
redundancy and failsafe (in the true sense of the
word) should apply. The minimum requirements for control systems are as
follows:-
Radio Systems.
The transmitter and receiver/s used must be tested and subject to
ongoing checks strictly in accordance with the
MAAA frequency Directive. This means that both the Tx and
Rx's have to be certified and that 10KHz operation is
not allowed. (MOP015 6.3.1.1)
Receivers.
It is recommended that two receivers, with separate power supplies,
switches and wiring, sharing each of the
primary controls be used. However the question of redundancy is a
contentious matter and should be discussed
with the inspector.
Some 2.4 GHz systems will not support two receivers
being used from one transmitter, in such
cases attention should be paid to battery redundancy and the
possibility of optically isolating Rx and servos. It is
recommended that Failsafe be used on the throttle channel to reduce
power in the event of loss of signal. For most
PPM systems an addon Failsafe would be needed. . (MOP015 6.3.1.2)
Batteries.
Battery redundancy is REQUIRED.
This may be provided by use of separate
batteries for each receiver or a common
supply using a battery backer system from a power board or other
similar device. Total battery capacity shall take
into account the number and power of servos, the required throws, the
size and speed of the model and the expected
number of commands to be exercised in flight. (MOP015 6.3.1.3)
Servos.
While the MOP specifies minimum torque required for servos on the
primary control surfaces, it is always best to
calculate the expected load. If you do the calculation, you may be
surprised!
Torque = surface area x chord x airspeed2 x torque coefficient.
Or try the calculator available on
http://www.geistware.com/rcmodeling/calculators.htm/
It is advisable to use a system that allows you to supply power to the
servos independently of the receiver/s.
Mechanical or other means of boosting torque supplied to a control
surface may be taken into account when
considering servo torque requirements on a control surface. This may be
in the form of a boost tab or similar system
that assists control surface movement. (MOP
15 6.3.1.4)
TEST FLIGHTS
A minimum of three (3) flights are required. (MOP015 6.5.1). Each
flight must be logged and at least the last two
test flights to be made must require no re-trim, repair, or major
adjustment to the airframe or radio before certification
(MOP015 6.3.2).
All pilots of Giant models shall have Gold Wings
endorsement for the type of aircraft being flown.
(MOP015 6.3.3).
ANNEXURES
Pre and During Construction/Assembly Inspection Assessment -
Form No. MAAA030
Checklist for
Inspection of a Model Aircraft - Form No. MAAA014
If you have any Building Tips
that you would like to share, please email Graeme Anderson.
web_manager@gmac.org.au
